In many fiber-optic systems application the demands on the light source are very stringent. Specifically, one needs good spectral purity in order to transmit high bit rate information over long distances. During laser modulation, a minimum level of spectral broadening necessarily occurs. Chirp, however, is an excess of spectral broadening beyond the spectral width required for modulation. Because different wavelengths propagate at different speeds in a dispersive medium, such as an optical fiber, the presence of significant chirp causes greater pulse spreading for a modulated optical signal transmitted across a fiber optic network.
Thus, reducing chirp has long been desired in optical communication. Low-chirp modulated laser sources are especially necessary in long-haul, high bit rate optical fiber transmission systems where chirp-induced pulse distortion reduces performance and range.
Two general approaches are typically used to modulate laser light: direct modulation and external modulation. In direct modulation, a laser such as a laser diode is directly modulated by an information signal to generate a modulated laser output. In external modulation, a modulator is used to modulate light from a laser source such as a laser diode. An information signal is then applied to the modulator rather than to the laser as in direct modulation. The frequency deviation (chirp) from external modulation is usually much lower than what can be achieved from a directly modulated laser, especially if a high extinction ratio of the transmitted light signal is required, despite the fact that chirp may arise from electrical and optical interactions between the laser and the modulator.
In external modulation, two different arrangements are commonly used for connecting the external modulator with respect to the laser source. In the first arrangement, the modulator and laser are disposed on separate, discrete substrates and in the second arrangement, the modulator and laser are fabricated as an integrated laser-modulator device on a single chip.
In the first arrangement, optical interactions between the laser and the modulator can be reduced by optically isolating the discrete laser and modulator from each other by means of e.g. an optical isolator arranged between the laser and the modulator, said optical isolator allowing light to travel in one direction only.
In the second arrangement, the laser and the modulator are fabricated on a single chip such that an interconnected laser-modulator is obtained. It is, however, complex and costly, if not impossible, to integrate an optical isolator with a laser and a modulator on a chip.
However, to reduce cost, complexity and size it is often desirable to use the second arrangement, i.e. the integrated combination of a laser and a modulator, and to reduce the optical interactions and thus the chirp by means of other approaches than using an optical isolator.
In FIG. 1 is shown, schematically, a top view of a typical integrated laser/modulator according to the prior art. A laser section 1 and a modulator section 3 are formed on a substrate 5 having a back facet 7 and a front facet 9. In order to obtain good performance using such an integrated arrangement, it is required that the internal reflexes on the chip can be reduced to very low levels since light reflected back to the laser will disturb the laser and cause chirp and other problems.
To reduce the back reflections in the modulator a so called window structure 11 is formed between the output end 13 of the modulator 3 and the front facet 9. The window region, being e.g. 15 microns long, consists typically of InP, and is arranged for free light propagation. Due to the divergence of the light output from modulator 3 the light reflected at front facet 9, which is coupled back into the waveguide, is reduced. Further, to reduce the reflectivity at front facet 9, typically an anti-refection (AR) coating 15 is formed thereon. It is important to keep a good coupling efficiency in the arrangement, and this complicates the procedures for reducing reflections back into the laser section.
The suppression of internal reflexes of such a conventional device, however, is in many instances not sufficient, which results in poor yield in the fabrication of such integrated modulator/laser devices.
The reflexes originate both from the on-chip modulator waveguide termination 13 and from residual reflections from the AR-coated front facet 9, which find their way back into the waveguide.